Low-Coordinate Mixed Ligand NacNac Complexes of Rare Earth Metals

We report the synthesis and characterization of two types of new mixed-ligand rare earth complexes: tetracoordinate (NacNacMes)Ln(BIANdipp) (Ln = Dy (1), Er (2) and Y (3)) and pentacoordinate (NacNacMes)Ln(APdipp)(THF) (Ln = Dy (4), Er (5) and Y (6)). The first three compounds were prepared by the reaction of [(BIANDipp)LnI] with potassium β-diketiminate. The salt metathesis of β-diketiminato-supported rare earth dichlorides (NacNacMes)LnCl2(THF)2 with sodium o-amidophenolate results in compounds 4–6. The crystal structures of complexes 1–6 were determined by single-crystal analysis. The combination of bulky monoanionic N-mesityl-substituted β-diketiminates with sterically hindered redox-active ligands led to the very low coordination numbers of rare earths and strong distortion of the chelate ligands.


Introduction
The combination of spectator ligands with different types of redox-active ligands constitutes a growing area of research in the coordination chemistry of rare earths over the past decades. The bulky N-aryl-substituted β-diketiminates or "NacNac" ligands with a general formula {ArNC(R)CHC(R)NAr} − are one of the most widely used ancillary ligands of post-metallocene series used for stabilizing rare earth complexes [1,2]. They readily form strong Ln-N (Ln = rare earth metal) bonds within six-membered chelating rings with bonding modes ranging from purely σ to a combination of σ and π donation. The easy-tunability of steric and electronic properties which can be implemented by an appropriate choice of starting β-diketones and anilines complete the utility of such ligands. Besides the classical role of spectator ligands, β-diketiminates behave like non-innocent ligands and may be involved in different transformations including redox reactions and metal-ligand cooperative activation of substrates [3].
The development of coordination chemistry of redox-active ligands with rare earths is of undoubted interest from the point of view of both fundamental science and applied research. The combination of a rare earth element with organic ligands which have extended redox properties and several reduction states will make it possible to obtain compounds possessing a number of useful chemical properties [4][5][6][7][8][9][10] as well as properties essential for the development of new magnetic materials [11][12][13][14]. In general, among such properties are the presence of different paramagnetic centers, several redox transitions, anisotropy of magnetic properties at the rare earth metal center, and modulating the magnetic behavior due to both the metal center and the organic redox-active ligand.
The most distinctive feature of redox active ligands (e.g., o-iminoquinones and alphadiimines) is the possibility of their redox transformations in the metal coordination sphere, which extends their reactivity to a great extent [15][16][17][18][19][20][21][22]. For a long time, o-iminoquinonato Molecules 2023, 28,1994 2 of 12 ligands have been used in the coordination chemistry of transition metals and a wide range of complexes have been synthesized to date [23][24][25]. As for main-group metals, a fair number of them are also known [26][27][28]. Notably, the combination "redox-active ligandmain group element" allows for the modeling of the reaction abilities of transition metals. For example, it has been shown that antimony(V) o-amidophenolates bind molecular oxygen in a reversible manner in mild conditions [29][30][31][32]. To the best of our knowledge, it is the first example of the main group metal complexes involved in reversible oxygen fixation. BIAN dipp , 1,2-bis[ (2,6-diisopropylphenyl)imino]acenaphthene, is a redox-active diimine ligand possessing the conformational rigidity of the diimine moiety and pronounced steric hindrances around nitrogen atoms. The unique reactivity of the main-group metal complexes with BIAN dipp ligand was shown by Fedushkin et al. in the examples of addition (in some cases, reversible) and activation of substituted alkynes, alkenes [33][34][35][36][37][38]. It would be very interesting to combine sterically hindered NacNac ligands with redox-active sterically hindered o-iminobenzoquinones and bis-iminoacenaphthenes in a coordination sphere of rare earth.
Here we report two types of mixed-ligand rare earth complexes with spectator monoanionic β-diketiminate ligands and sterically hindered dianionic diamides or o-amidophenolates. Two different synthetic routes were elaborated which are essentially based on the salt metathesis reactions. The first one includes a direct reduction of BIAN dipp with metallic rare earth excess in the presence of iodine to obtain in situ rare earth precursors [(BIAN dipp )LnI] (Ln = Dy, Er, Y), allowing further reaction with potassium salt of β-diketiminate (NacNac Mes )K affording four-coordinate complexes [(NacNac Mes )Ln(BIAN dipp )] (1)(2)(3). In contrast to this, five-coordinate complexes [(NacNac Mes )Ln(AP dipp )(THF)] (4-6) were obtained by the salt metathesis of disodium o-amodophenolate with β-diketiminate rare earth precursors [(NacNac Mes )LnCl 2 ]. Compounds 1-6 were characterized by standard analytic methods including single-crystal X-ray diffraction analysis which reveals unusually low coordination numbers of rare earths.

X-ray Structures
The molecular structures of complexes 1-6 in crystal state were determined by single crystal X-ray diffraction analysis (Figures 1 and 2, Figures S10-S15 of ESI). The dysprosium complex 1 crystallizes from n-hexane solution in the monoclinic space group P 2 1 /n, while the isostructural erbium 2 and yttrium 3 compounds crystallize from toluene in the triclinic space group P-1, and the unit cells contain one molecule of toluene per complex molecule. All complexes have a monomeric mononuclear structure in the solid state. The small differences in the structures of complexes are concerned with a weak variety of ligand conformations, distances, and angle values. The molecular structures of complexes 1-3 are shown in Figure 1 with the selected bond lengths and angles given in Table 1. respectively). In addition, the medium absorption bands near 1640-1650 cm −1 correspond to the stretch vibrations of conjugated C=C double bonds in the doubly reduced BIAN ligand (in 1-3) [33][34][35][36][37][38].

X-ray Structures
The molecular structures of complexes 1-6 in crystal state were determined by single crystal X-ray diffraction analysis (Figures 1 and 2, Figures S10-S15 of ESI). The dysprosium complex 1 crystallizes from n-hexane solution in the monoclinic space group P 21/n, while the isostructural erbium 2 and yttrium 3 compounds crystallize from toluene in the triclinic space group P-1, and the unit cells contain one molecule of toluene per complex molecule. All complexes have a monomeric mononuclear structure in the solid state. The small differences in the structures of complexes are concerned with a weak variety of ligand conformations, distances, and angle values. The molecular structures of complexes 1-3 are shown in Figure 1 with the selected bond lengths and angles given in Table 1.  The coordination sphere of rare earth atoms in complexes 1-3 consists of two bidentate chelating ligands-monoanionic β-diketiminate and dianionic BIAN dipp providing a coordination number of four. The Continuous Symmetry Measures (CSM) analysis [59,60] gave the description of the polyhedron as a tetrahedron as the best fit (Table S2). The index t4 proposed by L. Yang, D.R. Powell, and R.P. Houser [61] for the description of four-coordinate geometry was calculated to be 0.68 for complex 1, 0.69 for complex 2, and 0.72 for complex 3 (t4 = 1.00 for a perfect tetrahedral geometry, and t4 = 0.00 for a perfect square planar geometry). respectively). In addition, the medium absorption bands near 1640-1650 cm −1 correspond to the stretch vibrations of conjugated C=C double bonds in the doubly reduced BIAN ligand (in 1-3) [33][34][35][36][37][38].

X-ray Structures
The molecular structures of complexes 1-6 in crystal state were determined by single crystal X-ray diffraction analysis (Figures 1 and 2, Figures S10-S15 of ESI). The dysprosium complex 1 crystallizes from n-hexane solution in the monoclinic space group P 21/n, while the isostructural erbium 2 and yttrium 3 compounds crystallize from toluene in the triclinic space group P-1, and the unit cells contain one molecule of toluene per complex molecule. All complexes have a monomeric mononuclear structure in the solid state. The small differences in the structures of complexes are concerned with a weak variety of ligand conformations, distances, and angle values. The molecular structures of complexes 1-3 are shown in Figure 1 with the selected bond lengths and angles given in Table 1.  The coordination sphere of rare earth atoms in complexes 1-3 consists of two bidentate chelating ligands-monoanionic β-diketiminate and dianionic BIAN dipp providing a coordination number of four. The Continuous Symmetry Measures (CSM) analysis [59,60] gave the description of the polyhedron as a tetrahedron as the best fit (Table S2). The index t4 proposed by L. Yang, D.R. Powell, and R.P. Houser [61] for the description of four-coordinate geometry was calculated to be 0.68 for complex 1, 0.69 for complex 2, and 0.72 for complex 3 (t4 = 1.00 for a perfect tetrahedral geometry, and t4 = 0.00 for a perfect square planar geometry). The coordination sphere of rare earth atoms in complexes 1-3 consists of two bidentate chelating ligands-monoanionic β-diketiminate and dianionic BIAN dipp providing a coordination number of four. The Continuous Symmetry Measures (CSM) analysis [59,60] gave the description of the polyhedron as a tetrahedron as the best fit (Table S2). The index t 4 proposed by L. Yang, D.R. Powell, and R.P. Houser [61] for the description of four-coordinate geometry was calculated to be 0.68 for complex 1, 0.69 for complex 2, and 0.72 for complex 3 (t 4 = 1.00 for a perfect tetrahedral geometry, and t 4 = 0.00 for a perfect square planar geometry). Table 1. The selected bond lengths and angles in complexes 1-3 in crystals according to X-ray diffraction.
In contrast to 1-3, the coordination environment of lanthanides in o-iminoquinonato complexes 4-6 includes NacNac anionic ligand, dianion o-amidophenolate, and a coordinated THF molecule. According to the CSM analysis, the coordination polyhedron of 4-6 can be described either as a spherical square pyramid or a trigonal bipyramid (Table S3). In the former case, two N atoms and two O atoms lie in the square basal plane (Figure S16a), while in the latter, three N atoms lie in the trigonal basal plane ( Figure S16b). For compound 4·Hexane, one can also reasonably describe the polyhedron as a vacant octahedron. The molecular structures of complexes 4-6 are shown in Figure 2. The selected bond lengths and angles are given in Table 2. Table 2. The selected bond lengths and angles in 4-6 in crystals according to X-ray diffraction.

General
All manipulations of air-and moisture-sensitive materials were performed with the rigorous exclusion of oxygen and moisture in flame-dried Schlenk-type glassware either on a dual-manifold Schlenk-line, interfaced to a high vacuum (10 −3 mbar) line, or in an argon-filled MBraun or Korea Kiyon KK-021AS glovebox. THF and toluene were distilled under nitrogen from potassium benzophenone ketyl prior to use. n-Hexane was distilled under nitrogen over Na/K alloy prior to use. All solvents for vacuum line manipulations were stored in vacuo over Na/K alloy in resealable flasks. IR spectra were obtained in KBr pellet by means of a FT-801 Fourier spectrometer (Simex, Saint Petersburg, Russia) (complexes 1-3) and a Bruker FTIR Tensor 37 instrument by the attenuated total reflection method (ATR) (complexes 4-6) (Billerica, MA, USA). NMR spectra for 3 and 6 were recorded with a Bruker Avance 500 in benzene-d 6 . Chemical shifts were referenced to internal solvent resonances and were reported relative to tetramethylsilane ( 1 H and 13 C{ 1 H} NMR spectroscopy). Elemental analysis (C,H,N) was carried out with a Euro EA 3000 analyzer (Eurovector, Pavia, Italy). Starting materials [(NacNac Mes )LnCl 2 (THF) 2 ] [1], (NacNac Mes )K [40], [BianLnI] [62], IQ [68] and Bian dipp [69] were prepared according to literature procedures.

Synthesis of (NacNac Mes )Ln(Bian dipp ) (Complexes 1-3)
THF (15 mL) was condensed onto a mixture of BIAN dipp (100 mg, 0.2 mmol), iodine (26 mg, 0.1 mmol), and an excess of metallic rare earth (810 mg Dy; 840 mg Er; 450 mg Y, 5 mmol) with a continuous stirring. The mixture was stirred for 72 h at room temperature in order to reach an exhaustive reduction of BIAN dipp while the color was turning dark blue. The obtained dark-blue solution was added with stirring to a solution of (NacNac Mes )K (75 mg, 0.2 mmol) in THF (10 mL) resulting in the dark-blue solution and the gradual precipitation of KI. The mixture was stirred for 24 h at room temperature and then KI was filtered off. THF was evaporated under reduced pressure to dryness and the residue was dissolved in toluene (15 mL). The toluene solution was stirred at 90 • C for 24 h with the subsequent evaporation of the toluene to dryness. n-Hexane (10 mL) was condensed on the dark-blue residue and allowed to stay for a couple of days. In the case of (NacNac Mes )Dy(Bian dipp ) (1), dark-blue cubic crystals of 1 (103 mg, yield 52%) suitable for single-crystal X-ray diffraction were grown from this solution. In the case of (NacNac Mes )Er(Bian dipp ) (2) and (NacNac Mes )Y(Bian dipp ) (3), no crystals were obtained from the n-hexane solutions. Thus, n-hexane was evaporated to dryness and the residues were redissolved in toluene (10 mL). The toluene solutions were concentrated to approximately half their volume and allowed to stay for two weeks, affording dark-blue block-shaped crystals of 2·C 7

Synthesis of (NacNac Mes )Ln(AP dipp ) (4-6)
A solution of (AP dipp )Na 2 in THF (15 mL) (obtained by the exhaustive reduction of IQ (114 mg, 0.3 mmol) with the sodium excess) was added to a solution of [(NacNac Mes )LnCl 2 (T HF) 2 ] (213 mg Dy; 215 mg Er; 191 Y, 0.5 mmol) in THF (20 mL) resulting in the orange solution and the gradual precipitation of NaCl. The mixture was stirred for 48 h at room temperature and then NaCl was removed by filtration. Then THF was evaporated to dryness, and the orange residue was treated with two portions of n-hexane (2 × 10 mL) and subsequent evaporation to afford an orange solid. The solid was extracted with n-hexane in the two-section sealed ampoule. The concentration of the orange n-hexane extract by slow evaporation for two days afforded orange crystals of 4 0.5Hexane (185 mg, yield 62%), 5 (145 mg, yield 51%) and 6 (85 mg, yield 32%) suitable for single-crystal X-ray diffraction.

X-ray Diffraction
X-ray suitable crystals of 1, 2·Toluene, 3·Toluene, 4·0.5Hexane, 5, and 6 were covered with mineral oil (Aldrich, St. Louis, MO, USA), selected under a microscope, and mounted on the tips of thin glass fibers. The crystals were transferred directly to the cold stream of a Bruker X8 Apex diffractometer (at 150(2) K for 1, 2, 3) or a STOE IPDS 2 diffractometer (at 200(2) K for 4, 293(2) K for 5 and 150(2) K for 6). X-ray intensity data were collected using graphite monochromated Mo Kα radiation (λ = 0.71073 Å). The standard technique was used (ϕ-and ω-scans of 0.5 • frames). The crystal structures were solved using the SHELXT [70] and were refined using the SHELXL [71] programs with OLEX2 GUI [72]. Atomic displacement parameters for non-hydrogen atoms were refined anisotropically, with the exception of some disordered solvent molecules which were refined with DFIX, DANG, ISOR and RIGU restraints. Hydrogen atoms of organic ligands were placed in geometrically idealized positions and refined as riding on their parent C atoms.

Conclusions
We have prepared a series of low coordinate rare earth complexes (NacNac Mes )Ln(BIAN dipp ) (1-3) and (NacNac Mes )Ln(AP dipp )(THF) (4-6) (Ln = Dy, Er, Y) and shown a dependence of coordination environment of rare earth central metals on the bulkiness of the ligands. The salt metathesis reaction starting from either Ln-Cl or Ln-I derivatives proved to be a very prospective route to access the desired mixed-ligand complexes with various mono-and dianionic ligands. The neutral BIAN Dipp is able to be reduced with rare earth metals excess in the presence of iodine and further utilized in the reaction with potassium β-diketiminate, thus affording the unique four-fold coordinated complexes 1-3. The central rare earth atoms are coordinated only with two bidentate N,N-ligands without any solvate molecule. The less sterically hindered o-amidophenolate AP Dipp , formally with one oxygen atom instead of N-aryl group, enables a formation of five-fold coordinated complexes 4-6 by the reaction of β-diketiminato supported rare earth dichlorides (NacNac Mes )LnCl 2 (THF) 2 with sodium o-amidophenolate. Along with three nitrogen and one oxygen atom of two chelating ligands, the fifth position in the coordination sphere of rare earth atoms in 4-6 is occupied by a coordinated THF molecule.
Firstly, it should be noted that such a strong steric hindrance both of monoanionic N-mesityl substituted β-diketiminates and redox-active diimine or iminoquinone ligands leads to very low coordination and, in particular, extremely rare complexes of rare earths with severely distorted tetrahedral (non-solvate) and trigonal-bipyramidal geometry.
Secondly, the presence of two such bulky ligands in the coordination sphere of rare earths leads to the strong distortion of the ligands chelate cycles, which reduces the steric tension in the complexes as a whole.